1. Field of the Invention
This invention relates to a method of and apparatus for welding together metal components and is particularly, but not exclusively, applicable to the manufacture of vehicle drum brake shoes, each having an arcuate platform of which the convex surface ultimately supports a lining of friction material and the concave surface has at least one web extending perpendicularly thereto.
2. Description of the Prior Art
It is well known to weld components together using for example electrical or laser welding methods. One of the most common types of electrical welding is resistance/projection welding. This entails passing the components to be welded between top and bottom electrodes and producing either a continuous weld or a plurality of spot welds to connect the components together. Such a method, as exemplified in British Patent Specification No. 1462120, has hitherto been applied to the manufacture of drum brake shoes, the platform being connected to the web by a series of longitudinally spaced spot welds.
A considerable amount of heat is generated in the components during the welding operation and the components thereby become distorted. This is particularly disadvantageous in the manufacture of brake shoes because the distortion in the platform necessitates the coining or planishing of the latter to provide the correct degree of curvature to accept the usual lining of friction material. Moreover, it is important for at least the tips of the web to be dimensioned accurately and, because of heat distortion, the webs are usually blanked with a surplus of material at the tips and the webs require machining, after welding, to produce the correct dimensions. Further disadvantages become apparent in fluctuating weld quality due to irregular current flow which results from electrode wear, and in that the jig or other apparatus used for supporting the components during welding has to be electrically conductive to enable the welding currents to be applied. All of these factors lead to a relatively expensive product.
The use of laser radiation in welding together metal components is also known. An example of such a method is disclosed in U.K. Pat. No. 1562922 which relates to a method of welding together two flat flexible strips of metal. The two strips are brought together by a system of rollers to form a converging Vee between the strips such that facing reflective surfaces are provided. A focused laser beam is directed at the junction between the two strips so that laser radiation is reflected into the junction between the two strips and the weld established. The flexible nature of the two strips of metal and the symmetry of the arrangement means that little or no mechanical stresses are set up in the welded joint, and such stresses as are set up are relieved by the flexible nature of the metal strips.
Certain types of components are of "high restraint geometry" which, for the purpose of the Specification is defined as meaning a geometry such that the components cannot deform during welding to accommodate stresses set up in the components as a result of the welding process.